Security systems typically include a control panel with input/output interface circuits for connection to one or more sensor circuits and one or more output circuits. Each sensor circuit may include a plurality of sensors, each to detect a different alarm condition. Possible alarm conditions include opening of doors, excessive temperatures, detection of motion, and the like. Each output circuit may include one or more output devices to be activated on the occurrence of certain conditions. Possible output devices may include lamps, relays, strobes and the like. Each sensor circuit is connected to an input interface circuit, which measures parameters of the sensor circuit. Each output circuit is connected to an output interface circuit, which controls operation of any output devices on the output circuit.
Input and output requirements may vary from system to system, depending on system configuration, and particularly on the number of sensors and the number of output devices. Some systems may have a large number of sensor circuits, requiring a large number of input interfaces, but relatively few output devices. Other systems may have relatively few sensors, but numerous output devices such as lamps, relays, strobes and the like.
Given the variability of security system configurations, some security system panels include input/output interface circuits which are capable of functioning in both input and output modes.
Unfortunately, connection to an output circuit may cause damage to components of an interface. As a result, in conventional interfaces, components must be isolated in output mode. Unfortunately, some techniques for isolating components in output mode may introduce measurement errors in input mode. In addition, over-power conditions can cause damage to interface components.
Accordingly, there is a need for improved I/O interfaces which can be easily configured in either an input or an output mode, and which have protection against damage from overpower conditions.